A hallmark of atherosclerosis is the deposition of cholesterol esters in the arterial tissue. In mammals, biliary bile acid and cholesterol excretion account for about 85 percent of the cholesterol which is eliminated from the body. The competition which exists for lipoprotein uptake by liver and extrahepatic tissue is likely to pay an important role in the atherogenic process. The liver accounts for about 50 percent of LDL which is degraded and this process is stimulated via agents which augment bile acid synthesis. These data indirectly suggest a link between hepatic lipoprotein uptake and bile acid secretion. The major aim of this proposal is to examine the direct effects of hepatic lipoprotein uptake on cholesterol-bile acid metabolism using hepatocytes in culture. The ability to precisely control the environment of a hepatocytes in culture provides this study with an ideal tool. Our development of a sensitive (10 to the minus 12 g) quantitative measure of bile acids provides an accurate methodology to determine actual net rates of bile acid synthesis and secretion. Preliminary studies show that hepatocytes secrete cholic acid at rates which are about 30 percent of in vivo rates for 96 hrs. in culture. Additional new data suggest that hepatic cholesterol availability at least in part determines rates of bile acid synthesis. Our working hypothesis is that hepatic lipoprotein uptake is determined by (1) apolipoprotein composition (2) the expression of specific lipoprotein receptors and (3) the intracellular cholesterol pool. To examine putative regulators, detailed time courses will be determined of the effects of lipoproteins on hepatocyte cholesterol availability, the activities of ACAT, HMGCoA reductase, 7 Alpha hydroxylase, and rates of bile acid section. Lipoproteins will be isolated by column chromatography from control, cholesterol-fed, and hepatectomized rat plasma. Peripheral dog lymph isolated from dog hind limbs will also be used. To examine possible effects caused by the metabolic state of hepatocyte, lipoprotein particles will be added to hepatocytes obtained from adult, immature, cholestyramine-treated, and cholesterol-fed rats. Data will be analyzed with respect to primary effectors (rapidly invoked) and secondary effectors (more slowly invoked). The conclusions will aid our understanding of the potential role of the liver in maintianing cholesterol homeostasis via bile acid synthesis. Emphasis will be to elucidate mechanisms and regulators.